FORMATION OF OXYSTEROLS DURING OXIDATION OF LOW-DENSITY-LIPOPROTEIN BY PEROXYNITRITE, MYOGLOBIN, AND COPPER

Oxidation of low density lipoprotein (LDL) in the artery wall leads to the formation of cholesterol oxidation products that may result in cy totoxicity. Different mechanisms could contribute to LDL oxidation in vivo resulting in characteristic and specific modification of the chol esterol molecule. Alternatively, attack on cholesterol by chain propag ating peroxyl radicals could result in the same distribution of oxidat ion products irrespective of the initial pro-oxidant mechanism. To dis tinguish between these possibilities we have monitored the formation o f nine oxysterols during LDL oxidation, promoted by copper, myoglobin, peroxynitrite, or azo bis amidino propane. Regardless of the oxidant used, the pattern of oxysterol formation was essentially the same. The yields of products identified decreased in the order 7-oxocholesterol > 7 beta-hpdroxycholesterol > 7 alpha-hydroxycholesterol > 5,6 beta-e poxycholesterol > 5,6 alpha-epoxycholesterol except in the case of per oxynitrite in which case a higher yield of 5,6 beta-epoxycholesterol r elative to 7-oxocholesterol was found. No formation of cholestane 3 be ta,5 alpha,6 beta-triol, or the 24-,25-,27-hydroxycholesterols was see n. Concentration of 7-oxocholesterol levels in LDL was positively corr elated with the degree of protein modification. Endogenous alpha-tocop herol in LDL or supplementation with butylated hydroxytoluene prevente d oxysterol formation. Taken together these data indicate that the oxi dation of cholesterol and protein in LDL occur as secondary oxidation events consequent on the attack of fatty acid peroxyl/alkoxyl radicals on the 7-position of cholesterol, and with amino acids on apoB. Furth ermore, oxidant processes with atherogenic potential, such as peroxyni trire, copper, and myoglobin are capable of producing oxidized LDL con taining cytotoxic mediators.